1. Field of the Invention
The present invention relates to a machining apparatus for machining work pieces and a machining method thereof.
2. Description of the Related Art
FIG. 19 is a perspective view of a prior art printed circuit board drilling apparatus that is one of machining apparatuses, FIG. 20 is a front view showing a section near a spindle of the prior art printed circuit board drilling apparatus in FIG. 19 and FIG. 21 is a plan view of a table of the printed circuit board drilling apparatus in FIG. 19.
In FIG. 19, a table 1 of the printed circuit board drilling apparatus is supported by a guide not shown and is movable in an X-axis direction on a bed 20. A cross rail 21 is fixed to the bed 20 so as to straddle over the table 1. The cross rail 21 is provided with guides 23 on a front face thereof. The guides 23 support a cross slide 22 that is movable in a Y-axis direction along the guides 23 by a motor 24. The cross slide 22 is provided with guides 31 for supporting bases 30 on a front face thereof. The guide 31 is movable in a Z-axis direction by a motor 32. Each base 30 is provided with a saddle 33 fixed at a front face thereof and the saddle 33 is provided with a spindle 40 fixed therein.
As shown in FIG. 20, a drill 41 is held rotatably at the lower edge of the spindle 40. The spindle 40 also supports a pressure foot 42 fitted closely to the lower portion thereof in the Z-axis direction and pressed downward in the figure by an air cylinder not shown.
Printed circuit boards 2a to be worked, i.e., work pieces, are mounted in stacks on a surface 1a of the table 1 while each stack is laid upon a backing plate 2b and fixed together by two reference pins 3. The printed circuit boards 2a to be worked and the backing plate 2b will be referred to as a whole as a printed circuit board 2 hereinafter. It is noted that a phenol resin of 1 to 2 mm thick is often used for the backing plate 2b. 
As shown in FIG. 21, holes (shown as cutouts in the FIGS. 4 and 5 are formed in the table 1. A rectangular first clamp plate 7 is disposed in the hole 4 in which a V-shaped groove 6 is formed on the opposite side of the hole. While the length of the clamp plate 7 in the X-axis direction is substantially equal to the length of the side of the hole 4 in the X-axis direction, the length of the clamp plate 7 in the Y-axis direction is shorter than the surface-to-surface distance between the cylindrical surface of the first reference pin 3a contacting with the groove 6 on the side, in which the clamp plate 7 is disposed, and the surface 4b facing the groove 6 (the right-side surface of the hole 4 in the figure).
A rectangular second clamp plate 8 is disposed in the rectangular hole 5. While the length of the clamp plate 8 in the X-axis direction is substantially equal to the length of the side of the hole 5 in the X-axis direction, the length of the clamp plate 8 in the Y-axis direction is shorter than the distance obtained by subtracting the diameter of the second reference pin 3b from the length of the side of the hole 5 in the Y-axis direction. It is noted that the diameter of the first reference pin 3a is equal with that of the second reference pin 3b. Each of the clamp plates 7 and 8 is supported by a linear guiding unit 9 disposed at each bottom of the holes 4 and 5, which is composed of a bearing 9a and a track 9b so that respective upper surfaces of the clamp plates are leveled with the surface 1a of the table 1 as shown in FIG. 20 and are movable in the Y-axis direction by a driving means not shown.
Next, a machining procedure of the prior art printed circuit board drilling apparatus 100 will be explained.
Firstly, distances between the left-side surface 7a of the clamp plate 7 and the groove 6 and between the left-side surface 8a of the clamp plate 8 and the right inner-side surface 5a of the hole 5 are maximized by abutting right-side surfaces 7b and 8b of the clamp plates 7 and 8 against inner-side surfaces 4b and 5b of the holes 4 and 5 as shown by solid lines in FIG. 21. Next, after mounting the printed circuit board 2 on the table 1, the clamp plates 7 and 8 are moved left in the figure to abut the first reference pin 3a against the groove 6 by the left-side surface 7a of the clamp plate 7 and the second reference pin 3b against the left inner-side surface 5a by the left-side surface 8a. Then, the printed circuit board 2 is positioned in the Y-axis direction as a line O connecting centers of the two reference pins 3a and 3b becomes parallel with the X-axis and also in the X-axis direction by the groove 6.
The table 1 is moved in the X-axis direction while holding the printed circuit board 2 and the cross slide 22 is moved in the Y-axis direction in the above described manner to position the drill 41 above a machining section of the printed circuit board 2 (to determine machining position of the work piece). Then, the base 30 is moved in the Z-axis direction and the drill 41 is cut into the printed circuit board 2 while pressing the printed circuit board 2 by the pressure foot 42 to drill the printed circuit board 2.
The drilling apparatus 100 in the figure has four spindles 40, so that it can machine by four times of speed as compared to a printed circuit board drilling apparatus having one spindle 40 for example.
Although it is possible to improve machining efficiency by increasing a number of spindles, it requires a table having a large surface area. Furthermore, it is necessary to increase thickness of the table along the increase of the surface area of the table in order to keep rigidity of the table, thus increasing weight of the table. Then, Japanese Patent Application Laid-open No. 2002-050848 has proposed a printed circuit board drilling apparatus whose table weight is lightened by making the table, that is normally made of steel, of glass-fiber reinforced resin laminate.
Lately, it has become more and more important to downsize a driving source by lightening the machining table that is moved while mounting work pieces and to be able to position the work pieces quickly and accurately in order to increase efficiency, to save energy and to conduct fine-machining of the machining apparatus or the printed circuit board drilling apparatus described above in particular.
Though it is possible to lighten the weight of the machining table, as compared to a table made of steel, by using the material whose specific gravity is light, such as the glass-fiber reinforced resin laminate described above, it is unable to reduce the size of the table that moves during machining by a conventional arrangement that moves the whole machining table even if the table is lightened by using such material whose specific gravity is light. That is, this arrangement has been a drawback in lightening the table further.
There has been also a problem that the more the number of machining sections such as the spindles is increased, the more the area and thickness of the machining table increase, thus the weight increase rate of the table, that is moved during machining, is increased drastically.